In conventional speakers, paper pulp, metal, polypropylene, etc. are normally used as the diaphragm.
These conventional diaphragms have various drawbacks. Those made primarily from paper pulp are not moisture-resistant, fade under normal environment and degrade in aging. Those made from metal are hard on the surfaces and have high Young's modulus of elasticity but encounter harmonic distortion due to split vibrations and poor processability such as adhesiveness. Those made of synthetic resin such as polypropylene (PP) or formed polyethylene are low density, poor rigidity and poor heat-resistant.